The compound 1-imidazolylmethyl-2-naphthol is known. See, for example, Journal of Chemical Society, (1970), p. 1157-1161. However, use of said compound in conjunction with epoxy resins is not known.
Other types of imidazole catalysts and accelerators for use in conjunction with epoxy resins are known in the art and are widely used in many applications. Such imidazole compounds impart fast curing and a good balance of mechanical and thermal properties to the cured resins. See, for example, Lee and Nevill, SPE Journal, vol. 16, p. 315 (1960) and Farkas and Strohm, Journal of Applied Polymer Science, vol. 12, pp. 159-168 (1968).
The above-described known imidazoles, however, have certain drawbacks which limit their industrial use. For example, the known mixtures comprising polyepoxides and imidazoles (such as imidazole, 1-methylimidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, or 2-phenylimidazole) set up rapidly inasmuch as pot life at ambient temperature does not normally exceed 4-6 hours. Due to this short gel time, such compositions cannot readily be used in modem automated processes where a much greater stability is desirable, while at the same time, maintaining the ability of the resin to fast cure (i.e., a gel time of 0.5-5 minute) at the processing temperature.
Attempts have been made in the past to solve the above-described problem. For example, U.S. Pat. Nos. 3,356,645 and 5,001,212 teach a method of reducing the reactivity of imidazoles by formation of salts with organic and inorganic acids. But, in this case, only a slight increase in pot life is achieved, i.e., up to one week, which is still unacceptable for industrial standards.
Another approach is found in U.S. Pat. Nos. 3,638,007; 3,792,016; 4,101,514; and 4,487,914, which teach the formation of complexes of imidazoles with metal salts. However, in cases where a reasonable pot life is obtained in accordance therewith, it becomes necessary to increase the curing temperature to above 150.degree. C., which temperature is undesirable for industrial applications. Further, contamination of cured resins with the metals used to form the complex commonly causes an increase in water absorption which in turn has a negative effect on dielectrical properties.
Therefore, the principal object of the instant invention is to provide an imidazole-type catalyst for use in conjunction with epoxy resin systems which substantially eliminates the disadvantages encountered with prior art materials and the method of use of such catalyst.
It is another object of the present invention to provide an imidazole/resin system where polymerization is initiated within 1-2 minutes and is complete in 3-15 minutes at a temperature of from 110.degree.-150.degree. C. in order to facilitate automated processing of the resin compositions.
It is a further object of the invention to provide such an imidazole/resin system without any substantial adverse impact on the thermal and mechanical properties thereof.
Various other objects and advantages of this invention will become apparent from the following description.
It has now surprisingly been discovered that addition of 1-imidazolylmethyl-substituted 2-naphthol compounds to epoxy resins in accordance with the present invention improves the storage stability of the resins while maintaining a fast curing time at moderate temperatures in the range of 110.degree.-150.degree. C. Therefore, use of the imidazole derivatives in accordance with the present invention provides an epoxy resin system which can be stored at room temperature for prolonged periods of time (i.e., a longer pot life) without adverse effects on curing time.
Furthermore, addition of the instant imidazolyl naphthol compounds to the resin system surprisingly improves mechanical properties and decreases water absorption with improved retention of modulus under hot/wet conditions. These improvements allow, for example, a longer service life of composites, structural parts and adhesives made from epoxy resins, which greatly decreases costs associated with repair and replacement.